1. Field of the Invention
This invention relates to a backflow prevention device which prevents backflow of resin from the front of a screw to the surroundings of the screw in an in-line screw type injection molding machine.
2. Description of the Related Art
In an in-line screw type injection molding machine, used is an injection apparatus in which a screw is incorporated into a heating barrel. When the screw is rotated in a forward direction in the heating barrel, raw resin is introduced from a hopper into the heating barrel, and transferred forward by the screw. The raw resin is molten in the heating barrel while being stirred and heated, and sent to the front of the screw (this step is called “charging step”). After a molten resin of a predetermined amount is stored in the heating barrel, the screw is advanced and the molten resin is filled in a mold (this step is called “filling step”).
In the injection apparatus having the above structure, a backflow prevention device is provided between a screw tip (provided at a distal end of the screw) and the screw to prevent backflow of resin from the front to the surroundings of the screw. The structures of conventional backflow prevention devices are disclosed in, for example, Jpn. Pat. Appln. KOKAI Pub. No. 60-076321 and Jpn. Pat. Appln. KOKAI Pub. No. 63-260414.
FIG. 5 is a schematic diagram of a backflow prevention device disclosed in Jpn. Pat. Appln. KOKAI Pub. No. 63-260414. In the backflow prevention device, a check ring 20 is provided between a screw tip 21 and a screw 11. Teeth 22 are provided in a front end surface of the check ring 20. Grooves 23 with which the respective teeth 22 are engaged are provided in a rear end surface of the screw tip 21. Each tooth 22 is provided with an inclination on a side surface against which the engaged groove 23 is pressed when the screw 11 is rotated in the forward direction, such that the width of each tooth 22 is gradually narrowed toward the distal end thereof. The side surface reverse to the inclined side surface of each tooth 22 is formed vertical to the front end surface of the check ring 20. In conformity with this, each groove 23 is provided with an inclination of the same angle as that of the teeth 22 on a side surface which is pressed against the corresponding tooth 22 when the screw 11 is rotated in the forward direction, such that the width of each groove 23 is gradually narrowed toward the bottom thereof. A side surface reverse to the inclined surface of each groove 23 is formed vertical to the rear end surface of the screw tip 21.
By the above structure, the above backflow prevention device decreases the surface pressure in engaging portions between the teeth 22 and the respective grooves 23, to improve in the mechanical strength.
In addition, in the charging step, the check ring 20 is pushed back to the screw 11 side by a component generated by the inclination of the teeth 22, and thereby moves in a direction of closing the space between itself and the front end surface of the screw 11. Therefore, the resin path can be quickly closed when the subsequent injection step is started.
However, if suckback operation is performed after completion of the charging step, that is, in the case of performing an operation of slightly rotating the screw 11 in the backward direction, the component in the direction of closing the space between the check ring 20 and the front end portion of the screw 11 does not act on the check ring 20, and there is the fear that the resin path of the backflow prevention device is opened. Therefore, after completion of the charging step and before start of the filling step, the resin leaks from the surroundings of the screw to the front of the screw tip, and as a result there is a possibility of deteriorating uniformity of the resin amount filled in the mold.